Transducers, including microphones and condenser-type microphones, are typically utilized in hearing-aids. Generally, condenser microphones comprise a housing having an aperture which allows acoustic vibrations to communicate with the interior of the case, a diaphragm assembly comprising a diaphragm and support ring, and a charged plate. The diaphragm is typically spaced a set distance from the charged plate. The condenser microphones also comprise a printed circuit board (PCB) having a field-effect transistor (FET) connected to the PCB. These components are mounted within the housing. Quite often the housing comprises a cup-shaped housing having a base surface and an upstanding peripheral wall. The wall terminates at a distal edge defining an opening. In assembly of the microphone, the distal edge of the housing wall is rolled closed at the upper edge against the PCB.
One way in which condenser microphones operate is by allowing acoustic vibrations to enter the housing and allowing the diaphragm to respond thereto. The vibrating diaphragm causes a capacitance change between the diaphragm and the backplate.
It is understood that electret condenser microphones require the diaphragm assembly to be spaced from the backplate an exact predetermined distance, and, in one configuration, the diaphragm assembly must be electrically isolated from the housing. Additionally, it is understood that one of the backplate or the diaphragm is connected to suitable electronic circuitry to permit the electroacoustical interaction of the diaphragm and electret material on the backplate to provide an electrical signal representative of the acoustic vibration that entered the microphone housing. This signal is developed by the capacitance change between the diaphragm and the backplate, and is transferred to the PCB and ultimately converted to sound to be heard by a listener. As is known, the converse operation may be provided in that an electrical signal may be applied to the electret on the backplate to cause the diaphragm to vibrate and thereby to develop an acoustic signal which can be coupled out of the acoustic chamber.
One type of electret condenser microphone is disclosed in U.S. Pat. No. 5,097,515 (the '515 patent). The '515 patent discloses a condenser microphone composed of a fixed electrode and a movable electrode. The movable electrode is formed by a vibratory diaphragm secured to an end face of a tubular metal ring. The diaphragm has a metal film deposited on one of its surfaces. The fixed electrode in the '515 patent, however, is formed by at least a portion of an end wall of the cup-shaped housing of the microphone assembly. Accordingly, since the fixed electrode of the '515 patent is formed by the microphone housing, the microphone is extremely sensitive to any pressure or deformation of the microphone housing. Specifically, any pressure or deformation of the microphone housing changes the spacing between the cup (i.e., fixed electrode) and the diaphragm, thereby detrimentally effecting the performance of the microphone. Additionally, in order to isolate the diaphragm ring from the microphone housing, the '515 patent discloses coating interior of the microphone housing with a Teflon® coating. Unfortunately, coating the interior of the microphone housing with a Teflon® coating is a relatively difficult and expensive process.
Accordingly, a condenser microphone in accordance with the present invention provides an inexpensive and simple solution to eliminate the drawbacks of the prior condenser microphones.